A fixing unit in an image forming apparatus is designed as follows. That is, a heating roller has a core including a heater, and a layer formed around the core. The layer provides heat resistance and toner releasability. A pressure belt or a pressure roller is brought into contact with the heating roller with a certain load by use of a spring. Thus a nip portion is formed. Paper where an unfixed toner image has been formed is passed through the nip portion. Thus the toner image is fixed to the paper.
Due to such a configuration, when a paper jam occurs in the fixing unit, the pressure belt or the pressure roller has to be retracted from the heating roller so as to release the nip load applied to the heating roller in order to remove jammed paper. A mechanism using an eccentric cam driving unit to automatically move a frame engaging with he pressure belt or the pressure roller has been proposed as means for releasing the nip load (for example, Patent Document 1).
There has been used another means in which a lever is provided to come in contact with a member engaging with the pressure belt or the pressure roller so that the nip load can be released when a user operates the lever. A background-art example of a fixing unit using a pressure belt is shown in FIGS. 10A and 10B. FIG. 10A is a sectional view showing a state where the nip load has not been released, and FIG. 10B is a sectional view showing a state where the nip load has been released.
The fixing unit has a heating roller 1, a pressure belt 2, pressure arms 3, side plates 4, and so on. The heating roller 1 is an elastic roller in which a core 1a is coated with an elastic layer 1b made of silicon rubber or the like. The surface of the heating roller 1 is coated with PFA (tetrafluoride-perfluoroalkylvinyl ether copolymer) in order to secure releasability from toner. The heating roller 1 has a heater 5 in it. Toner is fused by heat from the heater 5. The heating roller 1 is rotatably supported on the side plates 4. The heating roller 1 is rotated in the arrow A direction by a not-shown driving unit composed of a motor.
The pressure belt 2 is, for example, made of a seamless polyimide film. The surface of the pressure belt 2 is coated with PFA in order to secure releasability from toner. The pressure belt 2 is suspended by a pressure unit 6 constituted by pressure members 6a and 6b and a pressure roller 6c. The pressure belt 2 is brought into contact with the heating roller 1 at an angle θ of contact by pressure springs 8 in cooperation with pressure stays 7 and the pressure arms 3 with a shaft 9 provided on the side plates 4 as a base point. Thus, the pressure belt 2 forms a nip (contact) portion h for fusing toner between the heating roller 1 and the pressure belt 2. At the same time, the pressure belt 2 rotates with the heating roller 1 due to the rolling friction of the heating roller 1.
Nip load releasing units 30 are placed in positions opposed to pressing portions of the pressure springs 8 and near tip portions of the pressure arms 3 respectively. The nip load releasing units 30 are supported on the side plates 4. Each nip load releasing unit 30 is constituted by an operating lever 30a and a support member 30b supporting the operating lever 30a rotatably in the arrow B direction. In the state of FIG. 10A where the nip load has not been released, the operating lever 30a is not in contact with a tip portion of the pressure arm 3, but the length of the pressure spring 8 pressing the pressure arm 3 toward the heating roller 1 is L1.
When paper jammed in the fixing unit is to be removed, a user rotates each operating lever 30a in the arrow B direction by an operating force F as shown in FIG. 10B. Thus, the operating lever 30a comes in contact with a tip portion of the pressure arm 3 so as to retract the pressure arm 3 and the pressure unit 6 around the shaft 9 and against the elasticity of the pressure spring 8. Due to such a structure, the pressure spring 8 is compressed so that the length thereof becomes L2 shorter than that when the nip load has not been released.
Patent Document 1: JP-A-10-10912
Such a background-art structure has the following problems. First, in the mechanism in which an eccentric cam driving unit is used to automatically move a frame engaging with a pressure belt or a pressure roller as disclosed in Patent Document 1, the number of parts increases due to the driving mechanism and so on. It is therefore difficult to make the apparatus smaller in size and lower in cost.
In the structure in which a user operates the operating levers 30a so as to release the nip load as shown in FIGS. 10A and 10B, the pressure springs 8 pressing the pressure arms 3 are displaced by the operating levers 30a in directions where the pressure springs 8 are compressed. Accordingly, the operating force of the user is so excessive that there occurs a problem in operating performance. It is generally desired that the operating force for a user to operate a lever or the like is not larger than 20 [N]. An operating force larger than 20 [N] does not only gives the user a sense of difficulty in operating, but also may injure user's hands. In the fixing unit using a pressure belt as shown in FIGS. 10A and 10B, however, the total nip portion contact load applied to the heating roller by the pressure belt reaches about 300-400 [N]. An excessive operating force is required for the operating levers 30a. 
In order to reduce the operating force F of the operating lever, the operating lever ratio (b/a) shown in FIG. 10B has to be increased, that is, the operating lever length b has to be enlarged. As a result, the operating levers 30a become so long that there occurs a great obstacle to miniaturization of the apparatus.